Attenuator impedance control

ABSTRACT

A variable attenuator system for providing variable attenuation of an imput signal in response to a control signal while automatically maintaining a predetermined characteristic impepdance comprising first and second series diodes and a shunt diode, each of the diodes including first and second electrodes; the first electrodes of the series diodes and the second electrode of the shunt diode being connected together at a common junction; the second electrode of the first and second series diodes being connted, respectively, to an input terminal and an output terminal; first and second impedance stabilizing circuits connected in parallel with the first and second series diodes, respectively, each of the stabilizing circuits including an impedance matching the predetermined characteristic impedance, and means for blocking flow of direct current and passing the signal to be attenuated received at the input terminals; a bias circuit for establishing a bias on the second electrodes of the series diodes; and a control circuit connected to the junction for controlling conduction of the shunt diode.

United States Patent Primary Examiner-Paul L. Gensler Attorney, Agent, or Firm-Joseph S. Iandiorio W liam Nov. 18, 1975 [5 ATTENUATOR IMPEDANCE CONTROL [57] ABSTRACT [75] Inventor: g fi i a Williams A variable attenuator system for providing variable atm erst tenuation of an imput signal in response to a control [73] Assignee: LRC, Inc., Hudson, NH, signal while automatically maintaining a predetermined characteristic impepdance comprising first and [221' Flled' June 1974 second series diodes and a shunt diode, each of the di- [21] App]. No.: 484,254 odes including first and second electrodes; the first electrodes of the series diodes and the second electrode of the shunt diode being connected together at a F8 common junction; the second electrode of the first [58] d "5 R 813A 307/237 and second series diodes being connted, respectively, le o are 307/263 to an input terminal and an output terminal; first and second im edance stabilizin circuits connected in par e W1 -t e 1rst an secon serles 1o es, respec- [56] References Clted tively, each of the stabilizing circuits including an im- UNITED STATES PATENTS pedance matching the predetermined characteristic 3,289,120 1 H1966 Anders et a1 333/81 R impedance, and means for blocking flow of direct cur- 3,346,805 10/1967 H ekimian 333/81 R X rent and passing the signal to be attenuated received 3,529,266 9/ 1970 Kmg 333/81 A at the input terminals; a bias circuit for establishing a 3,713,037 1/1973 Hop ferm 333/81 A bias on the sgfzond electrodes of the Series i and 5/1974 wllllams 307/263 a Control circuit connected to the junction for control ling conduction of the shunt diode.

5 Claims, 6 Drawing Figures ATT (db) VSERIES ISEFIES 1 78 7 A 0 A 76 e V|N V|N lN VSHJNT RSHUNT szmes (DJ-Kg 5O 80 82 0.3-

8215 o C0 Du ISHL NT IMPE 31mm;

US. Patent --Nov. 18,1975 Sheet2of3 3,921,106

US. Patent Nov. 18, 1975 Sheet 3 of3 3,921,106

Series Shunt Series Shunt DB resistance resistance DB resistance resistance l 289 4 343. 4 3. 8. 550 141. 9 2 576 2 ,l7l. 2 3. l 8. 829 137. 2 3 863 1, 447. 3 3. 2 9. 108 132. 7 4 1. 151 1,085. 3 3. 3 9. 386 128. 5 5 l. 439 868. l 3. 4 9. 663 124. 5 6 l. 726 723. 3 3. 5 9. 940 120. 8 7 2. 014 619. 8 3. 6 10. 22 117. 3 8 2. 301 542. 1 3. 7 10. 49 113. 9 9 2. 588 481. 7 3. 8 10. 76 110. 7 l. O 2. 875 433. 3 3. 9 ll. 04 107. 7 1. 1 3. 162 393. 8 4. 0 11.31 104. 8 1.2 3. 448 360. 8 5 14. 01 82. 24 l. 3 3. 735 332. 8 6 16. 61 66. 94 l. 4 4. 021 308. 9 7 19.12 55. 80 l. 5 4. 307 288. 1 8 21. 53 47. 31 l. 6 4. 592 269. 9 9 23. 81 40. 59 l. 7 4. 878 253. 8 1O 25. 97 35. 14 l. 8 5. 162 239. 6 11 28. O1 30. 62 1. 9 5. 447 226. 8 12 29 92 26. 81 2. 0 5. 731 215. 2 13 31. 71 23. 57 2. l 6. 011 204. 8 14 33. 37 20. 78 2.2 6. 299 195. 3 15 34. 90 18. 36 2. 3 6. 582 186.6 16 36. 32 16. 26 2. 4 6. 864 178. 7 17 37.62 14. 41 2. 5 7. 148 171. 3 18 38. 81 12. 79 2. 6 7. 428 164. 6 19 39. 91 11. 36 2. 7 7. 709 158. 3 40. 91 l0. l0 2. 8 7. 990 152 4 46. 93 3. 165 2. 9 8. 270 147. 0 49. 01 1. 000

ATTENUATOR IMPEDANCE CONTROL FIELD OF INVENTION This invention relates to a variable attenuator system and more particularly to a variable. attenuator system which automatically adjusts to maintain a predetermined characteristic impedance over the range of variable attenuation.

BACKGROUND OF INVENTION In conventional microwave circuits an attenuator is often located between the driver circuit and driven circuit. The driver circuit and driven circuit are designed to have matching impedances: both circuits have the same characteristic impedance. In order to minimize losses, the attenuator, too, is matched to this characteristic impedance. The attenuator, typically, consists of a T-pad or T-circuit including three resistors, two series resistors and a shunt resistor, connected at a common junction to resemble a T shape. In a T-circuit for any particular value of attenuation there is a specific resistance value for the shunt resistor to produce that attenuation and for each series resistor to maintain the characteristic impedance at that attenuation to match the impedance of the driver and driven circuits. Whenever the attenuation is changed the series resistors as well as the shunt resistor, must be replaced with resistors having other specific resistances, which combine with the resistance of the new shunt resistor to produce the desired characteristic impedance at the new attenuation level provided by the new shunt resistor. Tables are available of the values of the series resistors and shunt resistor required to maitain a specific characteristic impedance for specified attenuation. The lack of ease in quickly changing attenuation levels while maintaining impedance is not one of the best features of these systems.

Recently, in an attempt to supply a more flexible attenuator, diodes have been substituted for the resistors and series and shunt shaping circuits have been added to drive the diodes to the conduction level at which they exhibit the precise resistance required to maintain the impedance match at the particular attenuation level. However, the adjustment of the current level in any of the diodes effects the operation of each of the others: each time one is reset all must be reset. The variable feature therefore is only attained at the cost of an arduous, repetitious trimming process which must be performed for every change in the attenuation. In addition if the system is to work over a temperature range, temperature compensation must be introduced to each of the circuits to ensure that the complex, specific characteristic is maintained.

SUMMARY OF INVENTION It is therefore an object of this invention to provide an improved, simple, truly variable attenuator system which is capable of providing variable attenuation over a wide range of values and automatically adjusting to maintain a desired characteristic impedance throughout that range.

It is a further object of this invention to provide such an improved variable attenuator system having temperature compensation. I

The invention results from the realization that the series diodes in a T-circuit attenuator system can be biased to provide a characteristic resistance complementary to that of the shunt diode to produce a characteris tic matching impedance of any desired value over the range of attenuation.

The invention features a variable attenuator system for providing variable attenuation of an input signal in response to a control signal while automatically maintaining a predetermined characteristic impedance. There are first and second series diodes and a shunt diode. Each diode includes first and second electrodes. The first electrodes of the series diodes and the second electrode of the shunt diode are connected together at a common junction. The second electrode of the first and second series diodes are connected, respectively, to an input terminal and an output terminal. First and second impedance stabilizing circuits are connected in parallel with the first and second series diodes, respectively. Each of the stabilizing circuits includes an impedance matching the predetermined characteristic impedance and also includes means for blocking flow of direct current and passing the signal to be attenuated received at the input terminal. There is a bias circuit for establishing a bias on the second electrode of the series diodes and a control circuit connected to the junction for controlling conduction of the shunt diode.

DISCLOSURE OF PREFERRED EMBODIMENT Other objects, features and advantages will occur from the following description of a preferred embodiment and the accompanying drawings, in which:

FIG. 1 is a block diagram of a variable attenuator system with automatic impedance adjustment according to this invention;

FIG. 2 is a more detailed schematic diagram of the system of FIG. 1;

FIGS. 3A-C are idealized illustrations depicting the variation of certain parameters of the attenuator system of FIGS. 1 and 2;

FIG. 4 is a table of resistance values for symmetrical T-circuit having a 50 ohm characteristic impedance;

FIGS. 5A-E are idealized illustrations depicting the variation of certain parameters of the attenuator system of FIGS. 1 and 2; and

FIG. 6 is a more detailed schematic diagram of the circuit shown in FIG. 3. 7

There is shown in FIG. 1 a variable attenuator system 10 with automatic adjustment to maintain a predetermined characteristic impedance. Attenuator system 10 includes first and second series diodes l2 and 14 with their cathode electrodes connected to common junction 16 which is connected to the anode of shunt diode 18. Series diode 12 has its anode connected to input terminal 20 and series diode 14 has its anode connected to output terminal 22. Stabilizing circuits 24 and 26 are connected across series diodes l2 and 14, respectively. Shunt diode 18 has its cathode connected to ground and common junction 16 is connected to a control circuit 28 which controls the conduction of shunt diode l8. Biasing circuit 30 adjusts the resistance of series diodes 12 and 14. Blocking capacitors 32 and 34 provide DC isolation of attenuator system 10'.

In a preferred embodiment of attenuator system 10, FIG. 2, stabilizing circuit 24 includes a capacitor 40 and resistance 42; stabilizing circuit 26 includes a capacitor 44 and resistance 46. Biasing circuit 30 includes an adjustable power supply 50 with a filter capacitor 52 which provides a bias to the anodes of series diodes l2 and 14 through inductances 54 and 56, re-

spectively. Control circuit 28 includes inductance 58, filter capacitor 53, and transfer circuit 60.

The input signal at input terminal 20 is typically a microwave frequency while the control voltage V,-,, to transfer circuit 60 may be a video signal having a range of to volts. Inductances 54 and 56 are provided to prevent the microwave input signal at input terminal 20 from reaching adjustable power supply 50 and from establishing a second conduction path along line 62. Biasing circuit 30 may include inductances 54 and 56 without adjustable power supply 50 and capacitor 52. The biasing voltage would then be provided from an external source to the biasing circuit. Inductance 58 and capacitor 53 are also provided to isolate transfer circuit 60 from the microwave input signal appearing at common junction 16. Control circuit 28 may include inductance 58 without transfer circuit 60 in which case a control voltage would be supplied from some other source. Capacitors 40 and 44 are DC blocking capacitors to prevent the DC diode bias provided at the anodes of series diodes l2 and 14 from reaching the cathodes of their respective diodes through resistors 42 and 46, respectively. Resistors 42 and 46 have a resistance value approximately equal to the characteristic impedance desired for attenuator system 10. For example, if a desired characteristic impedance for attenuator system is 50 ohms resistors 42 and 46 each have a resistance of 50 ohms.

In operation it may be desirable to modulate the incoming microwave signal at input terminal with video control voltage V,-,,. This modulation is performed by the attenuating action of the output voltage from transfer circuit 60 applied to junction 16 at the anode of shunt diode 18. As the voltage supplied by transfer circuit 60 at junction 16 increases, diode 18 conducts the microwave signal more heavily shunting a greater portion of that signal to ground and greatly attenuating the microwave signal appearing at output terminal 22. Conversely, a decrease in voltage from transfer circuit 60 applied to junction 16 decreases the conduction of shunt diode l8 reducing the shunting to ground of the microwave signal appearing at junction 16 and causing an increase in the microwave signal at output terminal 22.

The attenuation 70, FIG. 3A, may be made linear with respect to the control voltage V,-,, if the voltage at junction 16, V,,,,,,,,, approximates the characteristic 72, FIG. 3B, with respect to V Characteristic 72 creates a current I,,,,,,,, through diode 18 which has the characteristic 74, FIG. 3C, with respect to V The voltage characteristic 72, FIG. 3B, may be obtained by using transfer circuit 60 as disclosed in US. Pat. No. 3,813,561, Voltage Control Switch Driver, Williams et al.

The impedance matching action of attenuator system 10 may be better understood with reference to FIG. 4 which is a table showing the resistance values for the series and shunt resistances for a T-circuit having a 50 ohm characteristic impedance as the power attenuation varies from 0.1DB to 40DB in increments of 0.1 from 0.1db to 4.0db; in increments of 1.0 from 4db to 20db and in increments of 10 from 20db to 40db. For example, for a power attenuation of Idb, shunt diode 18 should have a resistance of approximately 433.3 ohms while series diodes I2 and 14 should have a resistance of 2.875 ohms; for a power attenuation of IODB shunt diode 18 should have a resistance of 35.14 ohms while series diodes 12 and 14 should have a resistance of 25.97 ohms. The table in FIG. 4 thus shows the value of resistance that the diode must have for the various power attenuation levels in order to maintain a 50 ohm matching impedance at input terminal 20 and at output terminal 22. Since series diode 14 and stabilizing circuit 26 operate in the same manner as series diode 12 and stabilizing circuit 24, discussion of the latter will suffice to explain the operation of the former.

With the voltage, V,,,,,,,,, FIG. 3B, at junction 16 being variable from 0.3 volts to 0.7 volts and the power supply voltage set at approximately 1 volt the voltage across series diodes 12, v FIG. 5A, begins at the power supply voltage and decreases with increasing V,-,, as shown by characteristic 76. This causes the current through series diode 12, h to decrease with respect to increasing V,-,, as shown by characteristic 78, FIG. 58. While the resistance of shunt diode l8, R FIG. 5C, decreases from infinity to nearly zero, characteristic 80, with increases in V the resistance of series diode 12, R FIG. 5D, increases gradually until it reaches 50 ohms where it stabilizes as shown by characteristic 82, FIG. 5D, having steep portion 82b and flat portion 82a.

The invention results from the combination of four independent factors. The first factor is the realization that the resistance characteristic 80, FIG. 5C, of shunt diode 18 can be made to approximate the required shunt resistance for a particular value of characteristic impedance. The second factor is the use of a resistance equal to or approximately equal to the desired characteristic impedance in parallel with the series diode. For example, resistor 42 in parallel with series diode 12 has" a 50 ohm impedance. This causes the impedance between input terminal 20 and junction 16 to be maintained at effectivelySO ohms as indicated by the flat response of the upper portion of characteristic 82, FIG. 5D, even though the resistance of diode 12 is in excess of 50 ohms. The third factor is the realization that the resistance characteristic of series diode 12 from below 50 ohms, as shown by the lower portion 82b, FIG. 5D, approximates the series resistance, FIG. 4, from 40DB to 0.1DB. The fourth factor is the realization that the portion 82b of characteristic 82 can be made to optimally coincide with the series resistance required by adjusting the bias on the series diodes by means of a biasing circuit.

The combination of these factors causes the resistance of shunt diode 18 to follow the prescribed path for shunt resistance in a T-circuit.

Temperature compensation can be accomplished in a specific embodiment, FIG. 6, in which power supply 50 includes an operational amplifier which drives a current amplifier transistor 92 to provide an output voltage on line 62 to the series diodes l2 and 14. A resistor 94 limits the current which can be drawn from the power supply. One input to amplifier 90 comes from voltage divider circuit 96 including two resistors 98 and 100 in parallel with Zener diode 102. Resistance 98 is adjustable to control the output voltage on line 62. The other input to amplifier 90 is a feed back input from the output line 62 through a pair of diodes I04, 106 which are identical with series diode I2 and shunt diode 18, and which are in close thermal proximity to diodes l2 and 18. Input 108 to amplifier 90 is the reference input. Amplifier 90 operates to provide an output voltage with two feedback diodes 104 and 106 to maintain its other input 110 at the same value as reference input 108. An increase in temperature lowers the voltage required for a given resistance of diodes l2 and 18. Identical diodes 104 and 106 then also have lower voltage and therefore cause the voltage at input 110 to decrease so that the voltage on line 62 decreases and compensates for the lower voltage of diodes l2 and 18. This maintains the resistance characteristics of diodes 12 and 18 in a stable condition over a wide temperature range for example from -55to +125C. The input power connections can be made to positive and negative power supply through diodes 120 and 122, respectively.

Additional temperature compensation can be provided through an output stage 128 of control circuit 28 which may include an operational amplifier 130 having one input 132 for receiving a control signal and a second input 134 which is connected in a feedback loop through diode 136 identical to shunt diode 18. The output of amplifier 130 is fed through current amplifier transistor 138 and inductance 58 to junction 16. As the temperature increases, the voltage required for a given resistance of diode 136 decreases dropping the voltage to junction 16 and thereby compensating for the similar decrease in voltage of diode 18. Resistance 140 limits the current which can be drawn through output stage 128.

Resistor 142 in output stage 128 and resistor 144 in power supply 50 act as power limiting resistors; resistor 146 in output stage 128 and resistor 148 in power supply 50 provide paths for current to diodes 136 and 104 and 106, respectively. Diode 14 is compensated for whenever diode 12 is compensated for because they are connected in parallel and the voltage across each is the same.

Shunt diode 18 is twice temperature compensated because it provides a first path for current from power supply 50 through diodes l2 and 14 to ground and also supplies a path from output stage 14 through junction 16 to ground. Output stage 128 may be connected to positive and negative ground through diodes 150 and 152, respectively.

Other embodiments will occur to those skilled in the art and are within the following claims:

What is claimed is:

l. A variable attenuator system for providing variable attenuation of an input signal in response to a control signal while automatically maintaining a predetermined characteristic impedance comprising:

first and second variable resistance series diodes and a variable resistance shunt diode, each of said diodes including first and second electrodes, said first electrodes of said series diodes and second electrode of said shunt diode being directly connected together at a common junction;

said second electrodes of said first and second series diodes being connected, respectively, to an input terminal and output terminal;

first and second impedance stabilizing circuits connected in parallel with said first and second series diodes, respectively, each of said stabilizing circuits including an impedance matching the predetermined characteristic impedance, and means for blocking flow of direct current and passing the signal to be attenuated received at said input terminal;

a bias circuit for establishing a voltage bias on said second electrodes of said series diodes; and

a control circuit connected to said junction for providing a control voltage for controlling conduction of said shunt diode.

2. The variable attenuator system of claim 1 in which said bias circuit includes a power supply.

3. The variable attenuator system of claim 1 in which said bias circuit icludes a blocking impedance for impeding flow in said biasing circuit of the signal at said input terminal.

4. The variable attenuator system of claim 1 in which said control circuit includes a transfer circuit for generating a transfer voltage corresponding to the current characteristic of said shunt diode in response to a contol voltage.

5. The variable attenuator system of claim 1 in which said control circuit includes a blocking impedance for impeding flow in said control circuit of the signal at said input terminal. 

1. A variable attenuator system for providing variable attenuation of an input signal in response to a control signal while automatically maintaining a predetermined characteristic impedance comprising: first and second variable resistance series diodes and a variable resistance shunt diode, each of said diodes including first and second electrodes, said first electrodes of said series diodes and second electrode of said shunt diode being directly connected together at a common junction; said second electrodes of said first and second series diodes being connected, respectively, to an input terminal and output terminal; first and second impedance stabilizing circuits connected in parallel with said first and second series diodes, respectively, each of said stabilizing circuits including an impedance matching the predetermined characteristic impedance, and means for blocking flow of direct current and passing the signal to be attenuated received at said input terminal; a bias circuit for establishing a voltage bias on said second electrodes of said series diodes; and a control circuit connected to said junction for providing a control voltage for controlling conduction of said shunt diode.
 2. The variable attenuator system of claim 1 in which said bias circuit includes a power supply.
 3. The variable attenuator system of claim 1 in which said bias circuit icludes a blocking impedance for impeding flow in said biasing circuit of the signal at said input terminal.
 4. The variable attenuator system of claim 1 in which said control circuit includes a transfer circuit for generating a transfer voltage corresponding to the current characteristic of said shunt diode in response to a contol voltage.
 5. The variable attenuator system of claim 1 in which said control circuit includes a blocking impedance for impeding flow in said control circuit of the signal at said input terminal. 